This course covers basics of piezoelectric and capacitive micromachined ultrasonic transducers, i.e., PMUTs and CMUTs. At the end of this course, the attendees will leave with a good understanding of these two transducer types regarding their fundamental physics of operation, how they are typically fabricated, and the way they are used in practice.
This course is divided into two main sections. The first section starts with a short introduction into the various ultrasound transducer technologies, including piezoelectric and magneto-acoustic transducers. Then, PMUTs are introduced including their fundamental modelling, fabrication processes, and characterization techniques. The deflection equation of conventional unimorph planar PMUTs with arbitrary circular/ring electrode geometry is derived using the theory of plate vibration, and analytically solved using a Green's function approach. The equivalent circuit model is presented, which includes the influence of the residual film stress and voltage-induced piezoelectric diaphragm tensioning. Both analytical and numerical techniques are undertaken to optimize the performance of PMUTs such as their effective electromechanical coupling factor with respect to device geometry, electrode size, and excitation voltages. The micromachining processes used to realize PMUT arrays composed of various piezoelectric materials are outlined, and the major challenges to optimize the crystallinity of piezoelectric materials such as Lead titanate zirconate (PZT) and aluminium nitride (AlN) are highlighted. The various techniques used to characterize PMUT performance are detailed including x-ray diffraction (XRD), laser Doppler velocimetry (LDV), electrical impedance measurements, and microphone/hydrophone sound pressure level (SPL) testing, etc. The two recent efforts aiming at enhancing the effective electromechanical coupling factor of PMUTs are described including the static pressurization and dome PMUT approaches. This section will conclude by showing some of the medical images that were demonstrated by previously fabricated PMUT arrays.
The second part of this course is devoted to CMUTs, mainly covering their theory, modelling, fabrication, characterization, and integration with supporting electronics. We will start this section by introducing the parallel-plate capacitor approximation for a CMUT, which will allow us to derive some first-order fundamental formulas and an equivalent electrical circuit representation valuable in understanding the device operation. More sophisticated theoretical and finite-element based modelling approaches will be covered next. We will then talk about the two main CMUT fabrication processes commonly used today including their pros and cons. One of them is based on a sacrificial release process and the other employs a wafer bonding approach. Next, we will review various characterization tools, such as electrical input impedance and optical interferometer. Integrated circuits have proven fundamental to CMUTs in many applications. We will cover why that is the case, what the electronics requirements are, and how they are typically implemented. To help better understand the concepts, examples of CMUTs for various applications will be shown throughout this section.
Firas Sammoura is currently an Assistant Professor in the Department of Electrical Engineering and Computer Science at Masdar Institute of Science and Technology, Abu Dhabi. He is also a visiting scholar in the department of mechanical engineering at the University of California, Berkeley. Firas Sammoura received his B.E. in mechanical engineering from the American University of Beirut in June 2001 with high distinction. From 2001 until 2006, he was a graduate student researcher at the University of California at Berkeley. In May 2006, he earned his Ph.D. in the field of Microelectromechanical Systems (MEMS). His dissertation focused on building plastic millimeter-wave systems for radar applications at 95GHz. Prior to joining Masdar Institute, Dr. Sammoura was a visiting scientist in the MI/MIT cooperative program at the Massachusetts Institute of Technology where he started a new research program in the field of piezoelectric micromachined ultrasonic transducers (pMUTs) in collaboration with Prof. Sang-Gook Kim in the mechanical engineering department. From January 2007 until January 2011, he was senior device characterization engineer in the Advanced Development Group at the MEMS/Sensors division of Analog Devices in Wilmington, MA. He also worked as a student researcher with Hitachi Global Storage Technologies at the IBM Almaden Research Center from March 2004 until May 2006 where he did proprietary research in MEMS applications for the hard drive disk industry. Dr. Sammoura won the Spot Award for solving the stiction problem that plagued consumer inertial MEMS accelerometers. He has 8 pending and 7 patents in the field of microwave engineering and MEMS fabrication, design, and characterization.
Amin Nikoozadeh received the B.S. degree from Sharif University of Technology, Tehran, Iran, in 2002, the M.S. degree from Stanford University, Stanford, CA, in 2004, and the Ph.D. degree from Stanford University, Stanford, CA, in 2010, all in electrical engineering. For his Ph.D., he designed and developed fully-integrated ultrasound imaging catheters for forward-viewing intracardiac imaging using capacitive micromachined ultrasonic transducers (CMUTs). He joined the E. L. Ginzton Laboratory at Stanford University as a Research Associate in 2011. He has been a Senior Research Engineer at the E. L. Ginzton Laboratory at Stanford University since 2014. His past and present research interests include ultrasound imaging, image-guided therapeutics, MEMS, and analog circuit design, with a main focus on design, modeling, fabrication and integration of CMUTs. His current research focuses on the implementation of fully-integrated CMUT arrays for catheter-based ultrasound imaging, real-time volumetric ultrasound imaging using 2-D CMUT arrays with integrated electronics, high-intensity focused ultrasound (HIFU) therapy, low-intensity ultrasound for neurostimulation applications, photo-acoustic imaging, airborne ultrasound applications, and novel ultrasound transducer technologies.